Generally, a high-temperature exhaust gas discharged from a high-temperature gas generation source, such as an incinerator, a melting furnace or a reducing furnace, is subjected to a cooling treatment. This cooling treatment is intended to adjust a temperature of the high-temperature exhaust gas to a value suitable for use as a heat source for a boiler in a subsequent process, and/or lower a temperature of the high-temperature exhaust gas to a value equal to or less than an allowable temperature limit of a dust collector so as to allow the high-temperature exhaust gas to be released to ambient air after collecting dusts therein by the dust collector. The cooling treatment is performed by introducing a high-temperature exhaust gas discharged from the incinerator or the fusion furnace, into a cooling tower, and cooling the high-temperature exhaust gas by a wet process based on a scrubber or sprinkling of cooling water, within the cooling tower.
However, the process of cooling a high-temperature exhaust gas discharged from a incinerator, a fusion furnace, a reducing furnace or the like, by spraying cooling water within the cooling tower is highly likely to produce an adherent substance in the cooling tower. Specifically, since there are mixed ashes and solid dusts including a volatile (i.e., vaporizable) component, such as zinc or lead, and a melting component, such as alkali metal, oxide or chloride, in the high-temperature exhaust gas, the process of cooling this high-temperature exhaust will produce a liquefied substance of the volatile component and a solidified substance of the melting component, i.e., solid dusts, and cause a risk that the liquefied substance or the solid dusts adhere onto an inner wall of the cooling tower.
This phenomenon will be more specifically described by taking a rotary hearth furnace which is one type of a reducing furnace for use in producing reduced iron, as one example.
Firstly, one example of a reduced-iron production process using the rotary hearth furnace will be described step by step, with reference to FIG. 8 which schematically shows the structure of a rotary hearth furnace facility.
(1) An iron oxide (e.g., iron mineral or electric furnace dust) in powder form and a carbonaceous reducing agent (e.g., coal or coke) are mixed together, and pelletized. In this way, raw pellets are produced.
(2) The raw pellets are heated in a temperature range without ignition of a flammable volatile substance generated from inside the pellets. This heating eliminates water adherent to the raw pellets to produce dried pellets 14 illustrated in FIG. 8.
(3) The dried pellets 14 are supplied into a rotary hearth furnace 16 by an appropriate charging device 19, to form a pellet layer having a thickness equivalent to a size of about one or two pellets, on a rotary hearth 13.
(4) There is performed a combustion in a burner 17 arranged on an upper side of an inside of the furnace to radiation-heat the pellet layer to reduce it, thus metallizing the pellet layer.
(5) The metalized pellets are cooled by a cooling device 18. This cooling may be performed by directly blowing gas against the pellets or by indirectly cooling the pellets using a cooling jacket, for example. This cooling gives a mechanical strength tolerant to handlings during and after discharging the pellets. The cooled pellets, i.e., reduced iron pellets 15, are discharged outside the furnace by a discharge device 20.
(6) Immediately after the discharge of the reduced iron pellets 15, next dried pellets are charged by the charging device 19.
Reduced iron is produced by repeatedly performing the above process (see, for example, the following Patent Publication 1).
In the rotary hearth furnace for use in producing reduced iron, a high-temperature exhaust gas generated in the furnace is drawn from an exhaust-gas discharge area provided on a circumference of the rotary hearth furnace into a gas duct connected to a ceiling portion of the exhaust-gas discharge area, and introduced into an exhaust-gas treatment facility provided downstream of the gas duct to be treated therein.
This high-temperature exhaust gas, which contains dusts such as ash, will be finally released to ambient air after treated by a dust collector. The temperature of the high-temperature exhaust gas, which is generally equal to or greater than an allowable temperature limit of the dust collector, requires the high-temperature exhaust gas to be cooled down to a temperature allowing for the treatment by the dust collector before introduced thereinto.
As follows will be described a (cooling) treatment method and apparatus for the high-temperature exhaust gas according to an example of a conventional technique with reference to FIGS. 9 and 10. FIG. 9 shows a gas-cooling chamber disclosed in the following Patent Publication 2, wherein FIG. 9A is a partially cut-off side view, and FIG. 9B is a view taken along the arrowed line 9B-9B. FIG. 10 is a sectional view of a temperature control tower for use in a high-temperature exhaust-gas treatment method disclosed in the following Patent Publication 3.
In FIG. 9, a high-temperature exhaust gas HG is introduced into a lower portion of a cylindrical-shaped gas-cooling chamber 24 through a gas duct 26 to move upwardly therein while swirling, and then discharged from an upper portion of the cooling chamber 24. Cooling water is sprayed to the swirling flow of the upwardly-moving exhaust gas HG, and water droplets of the sprayed cooling water move upwardly together with the exhaust gas HG to cool the exhaust gas HG. The gas duct 26 is connected to an inlet 25 provided in the lower portion of the gas-cooling chamber 24 from a tangential direction of an outer peripheral surface of the gas-cooling chamber 24, to establish fluid communication with an internal space of the gas-cooling chamber 24. The gas-cooling chamber 24 has a top portion, which is provided with a gas outlet 27. A plurality of cooling-water spray nozzles 28 are mounted to a vertically-intermediate portion of the gas-cooling chamber 24, to spray the cooling water to the swirling flow of the upwardly-moving exhaust gas HG.
In the high-temperature exhaust-gas treatment method disclosed in the Patent Publication 3, a high-temperature exhaust gas discharged from a high-temperature gas generation source is blown into a temperature control tower 30 illustrated in FIG. 10. This temperature control tower 30 has an expanding stepped portion 31, 32 having a diameter which increases stepwise toward a downstream side of a flow direction of the high-temperature exhaust gas. Cooling water is sprayed to a gas flow of the high-temperature exhaust gas blown into the temperature control tower 30, to adjust a temperature of the blown-in high-temperature exhaust gas. The temperature adjustment separates solid dusts from the high-temperature exhaust gas, and the solid dusts are discharged outside the temperature control tower 30 to be collected. The exhaust gas discharged from the temperature control tower after completion of the temperature adjustment contains volatile/melting component dusts, which are collected by a bag filter.
In the above high-temperature exhaust-gas treatment method, forming a spiral-pattern swirling flow of a high-temperature exhaust gas introduced into the gas-cooling chamber enables reduction in size of the gas-cooling chamber and enhancement in dust collection function of the temperature control tower. However, it is still impossible to solve the problem that a mixture of cooling water sprayed into a cooling facility and melting components and dusts in a high-temperature exhaust gas adheres onto an inner wall of the facility as a solidified substance to cause operational difficulties.    [Patent Publication 1] JP 2001-181720A    [Patent Publication 2] JP 09-178367A    [Patent Publication 3] JP 2002-136826A